Antenna resonance

Antenna Resonance and Good SWR – Radios Guide

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Standing wave ratio is a prominent perception that characterizes how boss match lives betwixt a transceiver and antenna system. Recall that excellent match coincides to an SWR with greater attitude showing some degree of mismatch.We usually use omnipresence of low SWR as an intimation that an antenna is tuned perfectly and that is running fine. Occasionally you will listen radio hams say something like this “I know my antenna is resonant because the SWR is close to 1.” Or they valor claim that “A resonant antenna always has an SWR of 1.” So we want to tell you that all of remarks or proclamation you see are actually misleading you. So let’s take a look.

Absolutely, The SWR is all about impedance match. It tell us about how much good is your antenna or tell us about the matching to the transmitter of your antenna system that how well it is matched. As we know that our ham radio transmitters normally have 50 ohms output impedance so it means that a 50 ohms antenna can provide the perfect match and an SWR of 1. And everyone knows that in normal condition, we must have to use 50 ohms of the transmission line to preserve a consistent impedance throughout the system. And remember, how perfectly the antenna is working, SWR does not indicate this (directly). SWR only show the impedance of the antenna.


If you want to search about resonance, you will easily found a lot of definitions about it (resonance) but according to the Physics, it is something like this. Let’s take a simple review of resonance in the eye of physics that what physics says about it.

 A view of resonance in the eye of Physics.

A major perception for the antenna is the conception of resonance. One simple way to guess about the resonance of antenna is, we think you must have an experience with it, and that is “A swing.”

Consider, you are forwarding someone on a swing, where you can generate your pushes and from that, the swing moves back and forth with only on a small scale of force. The time period of the swing and your timed pushes are in resonance with one another. So “little pushes are an applied frequency that is perfectly timed to the swing’s period of oscillation.” 

swing resonanceHere, if we talk about our antenna resonance than it will define as “the interval of waving for an electrical charge which is running back and forth is normally resolved by antenna’s physical length. It is too easy to understand that if the height of antenna will exceed, the time interval of charges running will also increase just like “longer swing will take a long time to heave to and fro. And if the length of the antenna will be shorter, then the time travel from one end to another will be automatically reduced.

So in short, when the antenna is at resonance, its impedance is completely resistive. It means that voltage and current (Remember voltage and current are two different things) are in phase at the antenna feed point. By way of explanation, the main purpose of the antenna is to delivering the strength (efficiency) to the antenna that gets beamed. If we examine, then a part of antennas are improved than the other ones in the process of getting RF energy radiated.

antenna resonanceSome accepted representations of antenna resonance:

There are a lot of representations or in other words, you can say examples that will clear you everything related with antenna resonance. So let’s take a start from straightforward examples.

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A Dummy Load:

As we all know that a dummy load is inherently a resistor and we can examine the transmitters beyond absolutely by beaming signals by rehabilitation of antenna with a dummy load. For operating the ham radio, the dummy load will be of 50 Ohms, and yes it is a true fact that majority of ham radio systems conduct in 50 ohms. We know that a best and qualitative dummy load can cater the best or amazing impedance that rivalry with all kind of frequencies. Well, that’s not resonant. Because here, the ability or you can say efficiency of the antenna is 0 (Zero), this all is just because of it (Dummy Load) cannot beam any of the power. The reason is that, having only 50 ohms for all of the frequencies. So that was the perfect representation for understanding the antenna resonance with having low SWR but no beamed or radiated power.

Now the second example is

Classic center-fed half-wave dipole in free space

For the antenna, there is the impedance of 73ohm at the resonant frequency which is fully resistive or impervious. SWR can also be adjustable by taking the ratio of impedance to 50 ohms, which will give you SWR 73/50=1.5. (Well you want to calculate the SWR which is low than 50 ohms then you must have to apply this one SWR = 50/R.)

As we know that the dipole antennas are the best antennas or they work better. So the strength or efficiency of your antenna will be high and build upon on physical construction of the antenna. So note that SWR which is not comparable to 1 at resonance, it is a higher. Well, the SWR at 1.5 is a sign of perfect match and in general, it is expressed that is “FINE”.

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Ok now let’s take a glance at an antenna which is not nearer to 50 ohms at a resonance, the half – wave folded dipole antenna. Actually the half – wave folded dipole antenna has an impedance of 280 ohms at the resonate frequency so if you want to make a relation of an antenna with 50-ohm transmitter, then you must have to remember that the SWR will be calculated like this: 280/50 which is equal to 5.6. So that was the representation or a perfect example of high SWR. We want to tell you that at always remember that at the resonate frequency, this antenna will radiate or beamed efficiently and during the process it will make a difficult impedance to 50-ohm transmitter. So the impedance will be bad and you have to do a little bit struggle or may be more to deliver the power from the transmitter into the antenna.

The majority of antennas which we use generally, they are constructed to be close to 50 ohms which mean that the SWR will be 1 when they are resonant. So in this case, the SWR is the best guide to tell that Is Antenna is resonant or not? Well, Low SWR will never tell you about, how the antenna is working? Is it working well or not?

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A Dummy load has amazing SWR but it can’t radiate.


Question of the day…

How do you determine the length of an antenna and the size of a fractal antenna?

In an instructable, the formula for calculating antenna length was given as “length in meters =300 / frequency in MHz”.  The example given was for a Bluetooth antenna with a frequency of 2450MHz.  The antenna length was calculated as 300/2450 = 0.1225m = 12cm.  The author then divided it by 4 to make a “quarter wave” antenna length of 30mm.  Why divide it by 4 and what is the benefit of a “quarter wave” antenna.  What is the effect of the conductor leading to the antenna?  If the 30mm antenna is soldered to a 4mm lead, does that result in a 34mm antenna?  For a fractal antenna, should the 30mm length of wire be folded into a fractal shape or should the total fractal antenna be 30mm in width?  I have read somewhere that one of the benefits of a fractal antenna is that they are able to receive signals of the different wavelength.

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